A physiotherapist by profession, Shamsul gave up his practice to become a full time member of the Football Association of Singapore (FAS) in his role as referee's executive. Since becoming a referee on 1 January 1996, he has won both the 2005 and 2006 Asian Football Confederation Official of the year, becoming the first Singaporean to achieve this refereeing honour. He has also won the S.League Referee Of The Year award four times, in 1997, 1998, 1999 and 2001. He announced retirement in 2007[1] and in 2008 started to work with FIFA, first referee from Southeast Asia, as referee instructor.[2]

He is currently a FIFA Referee Instructor, an AFC Elite Referee Assessor and Recruiter and member of the FAS Referees Committee.[3]

He took charge of the group stage game between Trinidad and Tobago and Sweden, brandishing the first red card of the tournament to Trinidad and Tobago's Avery John. He also took charge of the group stage game between Mexico and Angola, sending off Angola's André Macanga, becoming the first referee at the tournament to produce two red cards. By taking charge of the Mexico–Angola match, Shamsul also became the first Singaporean to referee more than one match at a World Cup Finals.

He was however not one of the 12 referees to officiate the quarter-final matches onwards as he was sent home along with 14 other referees despite giving one of the most consistent performances [4] and escaping the scrutiny of the press which his fellow referees such as Valentin Ivanov and Graham Poll suffered from.